When E. coli is subjected to an energy source shift-down, protein synthesis is rapidly inhibited and a time-dependent disaggregation of polyribosomes and concomitant accumulation of complexed "monosomes" is observed. This redistribution is correlated with a specific decrease in the rate of peptide chain initiation and increased functional stability of mRNA. Neither the rate of peptide chain propagation nor the rate of protein turnover changes significantly. Electron microscopy has shown that the monosomes consist of single (70S) ribosomes lying at or near the 5'-ends of mRNA strands. Chemical analysis shows that these mRNA's retain 5'-triphosphate end groups (pppA and pppG). Preliminary results indicate that the complexes also retain about one mole of fmet-tRNA and we are engaged in isolation of this tRNA and verification of its acceptor activity and stoichiometry in the complexes. Measurements of ATP, GTP, CTP and UTP pools after shift-down indicate that the kinetics of changes in these pools are inconsistent with a role for any of these compounds as effector of reduced protein synthesis. Measurements of the kinetics of accumulation of ppGpp after shift-down show that this compound cannot be the primary inhibitor of protein synthesis in vivo. We are presently attempting to identify the putative inhibitor. Experiments are also underway to determine the mechanism by which arrest of protein synthesis at the initiation step leads to functional and physical stabilization of some, but not all, mRNA species.